Artificial disc

ABSTRACT

An implant device for surgical treatment of spinal disc damage or injury is formed as a resilient body comprising porous TiNi; the body is internally mobile such that it resiliently expands and contracts in response to variation in forces applied externally on the body.

TECHNICAL FIELD

The invention relates to a medical implant device for use in the surgeryof the spine.

BACKGROUND ART

Traumas or diseases of the spine result in damage to individualvertebrae, intervertebral discs, or combinations thereof Differenttechniques are used for the surgical treatment, according to theindications, among which is the prosthesis of lost or removed fragmentsand their stabilization within the spinal column, otherwise calledspondylodesis in surgical practice.

It is desirable to develop new and improved technical devices for spinalsurgery providing improved functional attributes, increase of successrate, and the further extension of their applications.

Existing approaches include a technique of posteriorspondylodesis [1.Russian Patent 2,076,6541] employing a device consisting of a set ofsupporting implants that are cylindrical in shape, a stratified implant,and a bracing device, the brace is made of TiNi alloy withthermomechanical shape memory.

Cylindrical implants are positioned into sockets created betweenadjacent spinous processes of injured vertebral bodies. These areemployed as supports between the vertebrae and serve to fix the injuredsegment of the spine.

The stratified implant is installed into a channel prepared in thecortical laminar layer and arched articular processes. Jointly and incontact with the cylindrical implants, it functions as a support andfixation device. The brace is used to brace the successive spinousprocesses and cylindrical implants, and it is also a dynamic elementassisting in the correction of possible scoliotic deformities of thespine in the injured area.

The disadvantage of this device is its structural complexity andaccompanying increased traumatism, the complexity of the operationrequired, and the high rigidity that results in the immobilization ofthe spinal segment.

A further device [2. Russian Patent 1,591,9781] for use in the surgicaltreatment of spinal injuries consists of a cylindrical rod made ofporous TiNi alloy and fixing elements in the form of undulating curvedclamps made of solid TiNi with shape-memory effect. In order to make thedevice compact and anatomically consistent, the fixing elements areinstalled into the longitudinal channels of the rod.

This device is used for the complete replacement of the vertebral body.It is fixed in place with fixing elements whose curved ends have beenpreviously introduced into sockets in the upper segments of the inferiorand lower segments of the superior adjacent intact vertebrae. During theinstallation, the shape-memory effect of the fixing elements isdeployed. The disadvantage of this device is its high rigidity, whichresults in the immobilization of the spinal segment.

Another device [3. Russian Patent 1,526,6751] for the surgical treatmentof the spine, which is used for partial or complete replacement of thevertebrae is illustrated in FIG. 1 and consists of a supporting elementof cylindrical shape made of porous TiNi alloy, with fixing projectionsat its ends, one of which is immobile, while the other, at the oppositeend, is movable in an axial direction and is provided with a spiralspring made of solid TiNi alloy with shape-memory effect. Whenperforming the prosthesis of the spinal segment, the ends of the deviceare installed into sockets prepared in the proximal intact bone portionof the spine. The fixing projections prevent potential luxation of thespine at the site of the implantation.

The disadvantage of this device is the rigidity of its structure, whichleads to the immobilization of the prosthetic segment.

DISCLOSURE OF THE INVENTION

The technical outcome of the present invention is the provision ofmobility in the prosthetic segment of the spine.

In accordance with the invention there is provided a non-fusion,intervertebral, implant device adapted to partially or completelyreplace an intervertebral disc in the spinal column of a humancomprising a resilient body, which body comprises porous TiNi, the bodybeing internally mobile such that the body resiliently expands andcontracts in response to variation in forces applied externally on saidbody.

In another aspect of the invention there is provided an improvement in asurgical method in which an intervertebral disc is replaced by animplant device, the implant device being a device of the invention asdefined hereinbefore.

DESCRIPTION OF PREFERRED EMBODIMENTS

i) Implant Device

The implant device of the invention may be generally referred to as anartificial disc, since it is to function as a partial or completereplacement for an intervertebral disc. However, while the implantdevice is employed for the function of an intervertebral disc it neednot have a disc shape.

The device is a non-fusion device, which means that the device will notcause a fusion of adjacent vertebral bodies, but will permit the spineto retain its flexibility by permitting movement between the adjacentspinal column members.

A device of the invention may suitably have a body with a structure oflayered sheets of porous TiNi, in which adjacent sheets of a pluralityof such sheets are in opposed, spaced apart facing relationship.

The structure is preferably an integral unitary structure; this may beachieved, in one embodiment by a body having the form of a rolled sheetof porous TiNi, the rolled sheet having a layered structure withadjacent winds or windings of the roll form being spaced apart.

Thus such a device for the surgical treatment of spinal injures containsa supporting element made of porous TiNi alloy; this element may be madein the form of a rolled sheet with a layered structure, preferably theopenings or spaces between the adjacent layers of the roll conform to aratio of 0.1 to 1.0:1 relative to the thickness of the sheet.

It will be understood that the sheet has a thickness such that it can bereadily coiled or rolled on itself to vary the diameter of the roll.

A cylindrical shape of the rolled form is recommended.

A flattened shape of the rolled form is also recommended.

In one preferred embodiment, a suitable porous TiNi alloy in the deviceof the invention has a porosity of 8 to 90%, and more especiallycomprises a porous body, in which the porosity extends throughout thebody. In particular, the body may be formed with a controllable andvariable porosity. On the other hand the invention applies also to otherporous TiNi such as that described in S.U. 1381764.

In preferred embodiments, employing the preferred porous TiNi referredto above, the porosity is defined by a network of interconnectedpassageways extending throughout the alloy; the network exhibits apermeability for fluid material effective to permit complete migrationof the fluid material throughout the network; this alloy is elasticallydeformable.

Preferably the porosity is at least 30% and preferably not more than70%.

Preferably the permeability is derived from capilliarity in the networkof passageways which define the porosity.

This capilliarity may be produced in the alloy by inclusion therein of alarge number of pores of fine size which interconnect to producecapilliary passages.

Capilliarity is advantageous in that it promotes migration of spinalbiological fluids into the network of passageways, and retention of thefluid material in the network, without the need to apply externalhydraulic forces.

In general the network has a coefficient of permeability of 2×10⁻¹³ to2×10⁻⁵, and the permeability is isotropic.

The capilliarity and the isotropic character are, in particular,achieved when the network defining the porosity comprises pores ofdifferent pore size, the pore size distribution preferably being asfollows:

Pore Size in Microns Quantity 10⁻²-10   5-15%  10-400 15-70%  400-100010-70% above 1000 remainder to 100%.

Suitable TiNi alloys for use in the invention are described in WO99/34845, published Jul. 15, 1999, the teachings of which areincorporated herein by reference.

Pore size is an important factor in tissue or biological aggregategrowth. At least some of the pores need to be of a size to permit thedevelopment or growth of biological aggregates synthesized from thecomponents of the biological fluid, for example, osteons.

Furthermore, if pore size is increased, the capillary effect decreases.

The medical objective of the vertebral prosthetics is the maximalrestoration of spinal functions, namely, the flexible shock-absorbingsupport of the organism.

The shape of the supporting element or device of the invention has agreat impact upon the outcome of the surgery. Suitably it may becylindrical. The cylindrical shape is more easily fabricated, andsimplifies the manipulations of the surgeon both when preparing thesocket for implant and during the fixation. In some cases anatomicalpecularities and the pathology of the spinal segment will call for anadaptive variation in the shape of the supporting element or device,namely, flattened like the intervertebral discs. The manufacture of suchelements is more elaborate, but recent developments in the technologyused for processing devices made of porous TiNi alloy have made thisrealizable.

ii) Titanium-Nickel

The porous titanium-nickel based alloy may suitably comprise 40 to 60%,by atomic weight titanium and 60 to 40%, by atomic weight, nickel to atotal of 100%; and more preferably 48 to 52%, by atomic weight,titanium, 48 to 52%, by atomic weight, nickel, less than 2%, by atomicweight, molybdenum, less than 2%, by atomic weight, iron and minor ortrace amounts of other elements, to a total of 100%. Desirably the alloycontains each of molybdenum and iron in an amount of more than 0%, byatomic weight and less than 2%, by atomic weight.

Nickel-titanium alloy has significant advantages, as compared with othermaterials, in biomedical applications such as the present implantdevice. In particular it displays a high level of inertness orbiocompatibility, it has high mechanical durability thus providinglongevity when employed in the fabrication of implants.

Bone tissue has an elasticity which renders it resilient to permanentdeformity when subjected to stress and vibrations. If material employedin an implant which contacts such bone tissue has differentcharacteristics from the bone tissue it will not meet the requirementfor biomechanical compatibility in an implant and longevity will beshort. The porous titanium-nickel alloy in the device of the inventionis found to display mechanical behaviour very similar to that of bonetissue, thus showing high biomechanical compatibility.

iii) Process

The porous titanium nickel of the implant device is preferably producedwith a controlled pore size distribution, as indicated above. Inparticular the porous titanium nickel may be produced in accordance withthe procedures described in the Russian publication “Medical Materialsand Implants with Shape Memory Effect” 1998, Tomsk University, p-460 to463, Gunther V. et al, the teachings of which are incorporated herein byreference. In summary, this alloy can be produced by powder metallurgymethod by means of the so-called SHS (Self-Propagating High-TemperatureSynthesis) method using two different processes:

-   -   Layerwise combustion SHS: Heat is generated by initial ignition.        Then thermal conductivity raises the temperature of the        neighbouring layers of the substance, thus causing reaction        within, and thus resulting in spatial displacement of the        reaction zone in the volume. The reaction takes place in a thin        layer called the combustion front.    -   Thermal shock SHS: In this process, the thermal shock is        performed by heating a mixture of various powders up to the        temperature at which the self supported chemical reaction and        heat release effect takes place. Due to the self-heating process        the mixture is heated up to higher temperatures, thus the        mixture of the powders is converted into alloy.

The two processes are explained more fully in the aforementionedpublication of Gunther V. et al and the aforementioned WO 99/34845published Jul. 15, 1999, both of which are incorporated herein byreference for their teachings of the processes for producing porousTiNi.

BRIEF DESCRIPTION OF DRAWINGS

The invention is illustrated in the accompanying drawings in which:

FIG. 1 is a prior art device for the surgical treatment of spinalinjuries

FIG. 2 is a device of the invention for the surgical treatment of spinalinjuries; and

FIGS. 3(a and b) is a Roentgenogram (X-Ray) of the device of theinvention implanted into the cervical segment of a patient's spine.

EXAMPLE

The possibility of achieving the stated technical outcome is illustratedby a test case of the clinical trial use of the implant in the RegionalClinical Hospital (RCH) of the City of Novosibirsk (case no. 2-3576).

Male patient D., 45 years old, was treated in the Department ofNeurosurgery of RCH for discogenic myelopathy, medial hernia of diskC₆-C₇, and tetraparesis.

Computer tomography confirmed posterior medial hernia of the disk C₆-C₇.

The patient underwent surgery for anterior interbody spondylodesis usingthe device of the invention. The device made of porous superelastic TiNialloy in the shape of a rolled sheet (FIG. 2) with the followingdimensions was used: length of the device 20 mm, diameter 18 mm,thickness of the sheet 0.4 mm, opening between the layers 0.2 mm.

The device was used in the following way. After total C₆-C₇ discectomy,a medial interbody socket with a diameter of 17 mm was created in thebodies of vertebrae C₆-C₇.

The device, preliminarily sterilized and elastically deformed by coilingthe roll to a diameter smaller than 17 mm, was installed into theinterbody socket. Under the effect of elasticity and due to theroughness of the surface of the porous structure, the device fixesitself in the osseous socket by uncoiling (FIG. 3). The wound wassutured. The cervical segment of the spine was fixed with a Philadelphiacollar. After 2 days the patient was transferred to active status.Positive dynamics in the form of a regression of tetraparesis wereobserved.

References:

-   1. Patent of the Russian Federation 6 A 61 B 17/56, 17/70 No.    2076654.-   2. Patent of the Russian Federation 5 A 61 F No. 1591978.-   3. Patent of the Russian Federation 4 A 61 B 17/60 No. 1526675.

1. A non-fusion intervertebral implant device adapted to partially orcompletely replace an intervertebral disc in the spinal column of ahuman comprising a resilient body, said resilient body comprising aporous TiNi sheet structure having a porosity defined by a network ofinterconnected passageways extending through the porous TiNi sheetstructure, said body being internally mobile such that the bodyresiliently expands and contracts in response to variation in forcesapplied externally on the body, said sheet being in a layeredarrangement in which adjacent layers are in opposed spaced apart, facingrelationship, with a ratio of spacing between adjacent layers tothickness of the sheet of 0.1 to 1:1.
 2. A device according to claim 1,wherein said structure is of a continuous sheet.
 3. A device accordingto claim 1, wherein said structure is an integral unitary structure. 4.A device according to claim 1, wherein said structure has the form of arolled sheet of porous TiNi in which adjacent windings of the rolledsheet are in said opposed spaced apart, facing relationship.
 5. A deviceaccording to claim 4, wherein said structure is cylindrical.
 6. A deviceaccording to claim 4, wherein said structure is in the form of acylinder flattened at opposed sides.
 7. A device according to claim 1,wherein said porous TiNi has a porosity of at least 30% and not morethan 70%, said network having a distribution of pore size as follows:Pore Size in Microns Quantity 10⁻²-10   5-15%  10-400 15-70%  400-100010-70% above 1000 remainder to 100%.


8. A device according to claim 7, wherein said network exhibits acoefficient of permeability of 2×10⁻¹³ to 2×10⁻⁵.
 9. A device accordingto claim 1, wherein said porous TiNi comprises 40 to 60%, by atomicweight, titanium, 40 to 60%, by atomic weight, nickel, less than 2%, byatomic weight, molybdenum less than 2%, by atomic weight, iron and minoramounts of other elements, to a total of 100%.
 10. A device according toclaim 1, wherein said porous TiNi comprises 48 to 52%, by atomic weight,titanium, 48 to 52%, by atomic weight, nickel, less than 2%, by atomicweight, molybdenum, less than 2%, by atomic weight, iron and minoramounts of other elements, to a total of 100%.
 11. In a surgical methodin which an intervertebral disc is partially or completely replaced byan implant device, the improvement wherein the implant device is anon-fusion intervertebral implant device adapted to partially orcompletely replace an intervertebral disc in the spinal column of ahuman comprising a resilient body, said resilient body comprising aporous TiNi sheet structure having a porosity defined by a network ofinterconnected passageways extending throughout the porous TiNi sheetstructure, said body being internally mobile such that the bodyresiliently expands and contracts in response to variation in forcesapplied externally on the body, said sheet being in a layeredarrangement in which adjacent layers are in opposed spaced apart, facingrelationship, with a ratio of spacing between adjacent layers tothickness of the sheet of 0.1 to 1:1.
 12. A method according to claim11, wherein said porous TiNi has a porosity of at least 30% and not morethan 70%, said network having a distribution of pore size as follows:Pore Size in Microns Quantity 10⁻²-10   5-15%  10-400 15-70%  400-100010-70% above 1000 remainder to 100%.


13. A method according to claim 11, wherein said porous TiNi comprises40 to 60%, by atomic weight, titanium, 40 to 60%, by atomic weight,nickel, less than 2%, by atomic weight, molybdenum, less than 2%, byatomic weight, iron and minor amounts of other elements, to a total of100%.
 14. A method according to claim 11, wherein said porous TiNicomprises 48 to 52%, by atomic weight, titanium, 48 to 52%, by atomicweight, nickel, less than 2%, by atomic weight, molybdenum, less than2%, by atomic weight, iron and minor amounts of other elements, to atotal of 100%.
 15. A device according to claim 1 wherein said networkexhibits a coefficient of permeability of 2×10⁻¹³ to 2×10⁻⁵, saidpassageways having a capilliarity effective to promote migration ofspinal fluids into the network and retention of such fluids in thenetwork without application of external hydraulic forces; and saidnetwork defining the porosity comprises pores of different pore sizes.16. A device according to claim 15 wherein said permeability isisotropic.
 17. A method according to claim 11 wherein said networkexhibits a coefficient of permeability of 2×10⁻¹³ to 2×10⁻⁵, saidpassageways having a capilliarity effective to promote migration ofspinal fluids into the network and retention of such fluids in thenetwork without application of external hydraulic forces; and saidnetwork defining the porosity comprises pores of different pore sizes.18. A method according to claim 17 wherein said permeability isisotropic.
 19. A method according to claim 11 wherein said structure isa unitary integral structure.